The United Nations refugee agency has joined calls for immediate evacuation of the remaining 1400 refugees and asylum seekers stranded on Nauru and Manus Island, citing a "collapsing health situation".

The UNHCR's appeal to the Australian government to end offshore processing follows warnings from global humanitarian body Médecins Sans Frontières about the "absolutely devastating" situation as dozens of refugees are knocked back for resettlement in the United States under President Donald Trump's "extreme vetting".

"Australia remains responsible under international law for those who have sought its protection," said UNHCR spokeswoman Catherine Stubberfield.

"In the context of deteriorating health and reduced medical care, Australia must now act to prevent further tragedy to those forcibly transferred under its so-called 'offshore processing' policy. UNHCR renews its call for refugees and asylum-seekers to be moved immediately to Australia, where they can receive adequate support and care."

Ms Stubberfield said more people had been evacuated from Nauru in September 2018 than in the previous two years combined, reflecting a "longstanding poor healthcare situation ... and a recent worsening of conditions, rather than any moderating of stance".